A boost converter is a type of forward converter where a smaller input DC voltage is increased to a desired level. A typical boost converter includes an inductor to which the input voltage is coupled that is in series with a diode connected to an output capacitor across which the load is connected. A transistor switch is connected to a node between the inductor and diode and ground to provide regulation of the output voltage. The control circuit for the transistor switch includes a comparator for sensing and comparing the output voltage of the converter to a voltage reference to generate an error voltage. This error voltage is then coupled to a duty cycle limited constant frequency pulse width modulator circuit (PWM). The PWM converts the error voltage into a control signal for controlling the timing of the on and off states of the transistor switch. When the transistor switch is on, the inductor current increases, storing energy in its magnetic field. When the transistor is off, energy is transferred via the diode to the load and the output energy storage capacitor. The transistor switch is operated at a high frequency relative to the resonance of the inductor capacitor network.
Drawbacks of such conventional boost converter circuits include the creation of switch voltage and current stresses and thus low efficiency power conversion. To overcome this drawback, boost converters have been proposed that provide soft switching, i.e., switching at low voltage and current stress across the transistor switch. A prior art boost topology for this type of converter is shown in FIG. 1.
As shown in FIG. 1, an input voltage V.sub.1 is converted into output power (V.sub.O) using a resonant network in addition to the conventional components of a boost converter. The resonant network comprises a resonant inductor L.sub.R, coupled in series with resonant diodes D.sub.1 and D.sub.2. An auxiliary switch S2 and resonant inductor L.sub.R are in series and are connected in parallel with main switch S1. A resonant capacitor C.sub.R connects the anode of resonant diode D.sub.1 to the anode of rectifier diode D.sub.O. In operation, control switches S1 and S2, are switched with complementary duty cycles, i.e., when one switch is on, the other is off. The current and voltage characteristics of auxiliary switch S2 and the resonant network of this topology are illustrated in FIG. 2.
A drawback exhibited by the boost topology of FIG. 1 is that, after the resonant inductor L.sub.R has reset, the voltage across auxiliary switch S2 drops from a higher voltage level (of approximately 400V) to a lower voltage level (of approximately 200V). The voltage drop across auxiliary switch S2 is illustrated by Trace A of FIG. 2. This voltage drop causes a current to flow in the resonant diodes D.sub.1 and D.sub.2 prior to auxiliary switch S2 turning on. The current flow through the resonant diodes is illustrated by Trace B of FIG. 2. The voltage drop across auxiliary switch S2, coupled with the resonant diodes conducting current before the auxiliary switch S2 turns on, results in a large amount of stress being placed on auxiliary switch S2 and associated power losses associated with the turning on of auxiliary switch S2. Increased EMI noise also results. The power lost through the auxiliary switch reduces the efficiency of the boost converter.